<head>
    <style>
        .left {
            width: 100%;
            float: left;
            background: #f00;
            margin-right: -200px;
        }
        
        .right {
            float: left;
            width: 200px;
            background: #0f0;
        }
    </style>
</head>

<div class="left">
    <p>hello</p>
</div>
<div class="right">
    <p>world</p>
</div>